Oil warming strategy for transmission

ABSTRACT

A method is provided for increasing a temperature of oil in a continuously variable transmission during engine start-up, the transmission including a fluid pump and motor, and a mechanical transmission. The method includes starting the engine, maintaining the fluid pump at substantially zero displacement, and heating the transmission oil by relative rotation of a first clutch disc and first clutch hub of a first clutch assembly and by relative rotation of a second clutch disc and second clutch hub of a second clutch assembly. The method further includes maintaining a substantially zero net torque from the transmission during the heating of the oil by the first and second clutch assemblies.

PRIORITY

This is a continuation of U.S. patent application Ser. No. 11/305,185,filed Dec. 19, 2005, now U.S. Pat. No. 7,410,446 which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to an oil warming strategy fora transmission, and more particularly, to an oil warming strategy for ahydromechanical transmission.

BACKGROUND

Hydraulic fluid, e.g., oil, in a transmission typically has a lowtemperature, e.g., −10° C., under cold start conditions. At such a lowtemperature, there is increased flow resistance owing to an increasedviscosity of the oil. As a result, the magnitude of the load imposed onthe engine is substantially great. Furthermore, stroking a variabledisplacement pump in the transmission to maximum displacement under suchconditions can damage the pump. Specifically, because maximumdisplacement from the pump is required to engage the clutches, at zeroground speed the machine cannot move until the oil is warm enough tostroke to maximum displacement.

Methods for warming oil in a transmission under cold start conditionsare well known. One method of warming the oil in a transmission isdescribed in U.S. Pat. No. 5,115,694 (the '694 patent) issued to Sasakiet al. The '694 patent describes a warming-up operation for an engine ina transmission that starts in cold-conditions. Hydraulic fluid, e.g.,oil, is supplied from a pump driven by the engine to a hydraulicallyactuable coupling of the transmission, i.e., clutches and brakes, forvarying a gear ratio of the transmission. If the temperature of the oilin the transmission is below a predetermined threshold, then thehydraulic pressure of the oil supplied by the pump is decreasedtemporarily to a minimum value. Since the magnitude of the line pressureis temporarily decreased, the load on the engine imposed by the pump isdecreased and the oil may be subsequently warmed up. After temporarilydecreasing the line pressure to warm the oil, the line pressure may beincreased to a maximum value.

Although the warm-up strategy of the '694 patent may warm the oil in thetransmission under cold start conditions, the '694 patent warms the oilby decreasing line pressure in the oil supply, thereby decreasing theamount of oil circulating in the transmission.

The disclosed system is directed to overcoming one or more of theproblems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure is directed to a method forincreasing a temperature of oil in a continuously variable transmissionduring engine start-up, the transmission including a fluid pump andmotor, and a mechanical transmission. The method includes starting theengine, maintaining the fluid pump at substantially zero displacement,and heating the transmission oil by relative rotation of a first clutchdisc and first clutch hub of a first clutch assembly and by relativerotation of a second clutch disc and second clutch hub of a secondclutch assembly. The method further includes maintaining a substantiallyzero net torque from the transmission during the heating of the oil bythe first and second clutch assemblies.

In another aspect, the present disclosure is directed to a method forincreasing a temperature of oil in a continuously variable transmissionduring engine start-up, the transmission including a fluid pump andmotor, and a mechanical transmission. The method includes starting theengine, incrementally increasing the displacement of the fluid pump inresponse to a first set of predetermined conditions, and incrementallyincreasing the speed of the engine in response to a second set ofpredetermined conditions. The method further includes maintaining asubstantially zero net torque from the transmission during theincremental increasing of the displacement of the fluid pump and speedof the engine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed workmachine;

FIG. 2 is a schematic view of an exemplary disclosed hydromechanicaltransmission of the work machine of FIG. 1;

FIG. 3 is a schematic view of the hydromechanical transmission of FIG. 2with a first synchronizer at a low-reverse position and a secondsynchronizer at a low-forward position;

FIG. 4 is a schematic view of a control system of the hydromechanicaltransmission of FIG. 2;

FIG. 5 is a flow chart illustrating an exemplary disclosed oil warmingstrategy for the hydromechanical transmission of FIG. 2;

FIG. 6 a is a graph illustrating engine speed versus time for anexemplary disclosed oil warming strategy for the hydromechanicaltransmission of FIG. 2; and

FIG. 6 b is a graph illustrating pump displacement versus time for anexemplary disclosed oil warming strategy for the hydromechanicaltransmission of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary work machine 8 having an exemplarycontinuously variable transmission. The work machine 8 may embody afixed or mobile machine that performs some type of operation associatedwith an industry such as mining, construction, farming, transportation,or any other industry known in the art. For example, the work machine 8may be an earth moving machine such as an excavator, a dozer, a loader,a backhoe, a motor grader, a dump truck, or any other earth movingmachine.

FIGS. 2 and 3 illustrate schematic views of the exemplary continuouslyvariable transmission 10 in two different settings, which are describedbelow. The components of the transmission 10 are surrounded by oil, andthe oil can be circulated throughout the transmission 10 using a pump(not shown).

The continuously variable transmission may be a hydromechanicaltransmission 10 having a mechanical transmission 16 and a hydraulicvariator in the form of a hydrostatic transmission (pump and motor) 14.An engine 12 drives the hydromechanical transmission 10, and may be aninternal combustion engine, however, it may be any kind of devicecapable of powering the continuously variable transmission as describedherein. The engine 12 outputs to the hydrostatic transmission 14 and themechanical transmission 16 through an input member 18.

The input member 18 provides split power to the hydrostatic transmission14 and the mechanical transmission 16 through first and second fixedinput gears 20 and 22. The term “fixed” may be understood as beingintegral with, permanently attached, interconnected through a splinedconnection, or fused by welding, for example, or by any other meansknown to those having ordinary skill in the art.

The hydrostatic transmission 14 includes a variable displacement pump23, such as a fluid pump, drivingly connected to the engine 12, througha hydrostatic transmission input gear 24, and a motor 26, which outputsthrough a hydrostatic transmission output gear 28 to the mechanicaltransmission 16. The motor 26 may be variable displacement or fixeddisplacement. One skilled in the art will realize that the hydrostatictransmission 14 may also be embodied as an electric generator andelectric motor, or other device capable of providing input power,without departing from the scope of the present invention.

The mechanical transmission 16 includes a planetary arrangement 30,first and second output members 32 and 34, first, second, and thirdsynchronizing assemblies, or synchronizers, 36, 38, and 39 and first andsecond disc clutch assemblies 40 and 42. It is noted that although theillustrated embodiments show the use of synchronizers 36, 38, and 39, itis anticipated that the synchronizers 36, 38, and 39 may be substitutedfor other known engaging means, such as friction disc type clutches.

The planetary arrangement 30 includes first and second axially alignedplanetary gear sets 44 and 46, and a planetary output shaft 48. Eachplanetary gear set 44 and 46 includes a sun gear 50, a carrier 52, and aring gear 54, as is customary. The planetary output shaft 48 includes aninternal shaft 56 and a sleeve 58, such as a hollow member or hub, whichis supported by the internal shaft 56. Both the internal shaft 56 andthe sleeve 58 exist in axial alignment with each other. The internalshaft 56 connects to the sun gears 50 of the first and second planetarygear sets 44 and 46. The sleeve 58 outputs from the carrier 52 of thesecond planetary gear set 46 through a first planetary output gear 60.The internal shaft 56 outputs from the sun gears 50 of the first andsecond planetary gear sets 44 and 46 through a second planetary outputgear 62 and through an auxiliary drive gear 63. The first and secondplanetary output gears 60 and 62 are fixed to the planetary output shaft48, while the auxiliary drive gear 63 rotates thereon.

The first and second output members 32 and 34 are positioned parallel tothe input member 18 and the planetary arrangement 30. The first outputmember 32 includes a first low-speed reduction gear 64 and a firsthigh-speed reduction gear 66. The second output member 34 includes asecond low-speed reduction gear 68 and a second high-speed reductiongear 70.

The first and second synchronizers 36 and 38 are fixed to first andsecond hubs, or rotating members 72 and 74, respectively, and operate torotate about the corresponding first or second output member 32 and 34on at least one bearing or the like (not shown). The first and secondsynchronizers 36 and 38 are three-position synchronizers adapted to movefrom a neutral position to either of two positions, dependent on apreferred speed and direction. The third synchronizer 39 is fixed to theinternal shaft 56 of the planetary output shaft 48, permanently, orthrough a coupling such as a spline, and moves from a neutral positionto an engaged position.

Each rotating member 72 and 74 includes a rotatable clutch disc 78 and80 fixed to an end of the rotating member 72 and 74, which may be“clutched” or selectively retained by an engaging clutch hub 82 and 84,which generally overlays the clutch disc 78 and 80, as is customary. Arelatively small distance separates the clutch discs 78 and 80 from therespective clutch hubs 82 and 84. Together, the clutch discs 78 and 80and clutch hubs 82 and 84 embody the first and second clutch assemblies40 and 42. In one embodiment, the clutch assemblies 40 and 42 are knownhydraulically-engaged and spring-disengaged rotating frictional clutchassemblies which may be selectively engaged to connect one of the firstor second output members 32 and 34 to a final output member 86.

The low-speed and high-speed reduction gears 64, 66, 68, and 70 areconfigured to freely rotate about the first and second output members 32and 34 while disengaged. Roller bearings (not shown) on the first andsecond output members 32 and 34 support the low-speed and high-speedreduction gears 64, 66, 68, and 70. When either of the first or secondsynchronizers 36 and 38 is engaged with either of the low-speed orhigh-speed reduction gears 64, 66, 68, and 70, the first or secondrotating member 72 and 74 rotates at the same revolutions per unit oftime as the engaged low-speed or high-speed reduction gear 64, 66, 68,and 70. Likewise, when the third synchronizer 39 is engaged with theauxiliary drive gear 63, the auxiliary drive gear 63 rotates at the samespeed as the internal shaft 56 of the planetary output shaft 48, whichdrives an auxiliary output gear 87, which is fixed to the first outputmember 32.

First and second output shaft gears 94 and 96 fixed to the first andsecond output members 32 and 34 intermesh a final drive gear 98 of thefinal output member 86.

As is customary, the input member 18, planetary output shaft 48, firstand second output members 32 and 34, and final output member 86 aresupported within a transmission housing (not shown) and rotate aboutbearings, or the like, (not shown) held within the housing.

FIG. 4 illustrates a schematic view of an exemplary control system 110for the transmission 10. A temperature sensor 112 is provided formonitoring the oil temperature, e.g., near the variator and upstreamalong an oil supply line for the pump 23. Signals indicating the oiltemperature are transmitted from the temperature sensor 112 to a devicefor monitoring an oil warming rate, such as a processor 114 connected toa memory 116.

The processor 114 may be connected to a control device 118 for sendingsignals or commands to various components of the transmission 10, e.g.,the pump 23 and the engine 12, in response to the oil temperature signalreceived from the processor 114. For example, the control device 118 maysend a signal to the engine 12 to increase an engine speed. The controldevice 118 may send a signal to the pump 23, which rotates with theengine 12, to increase a pump displacement.

The memory 116 may store data relating to the increment by which enginespeed or pump displacement is increased. The memory 116 may also storedata for determining a maximum value for increasing the engine speedwithout damaging the transmission (or highest safe engine speed, vsafe),a maximum value for increasing the pump displacement without damagingthe transmission (or highest safe pump displacement, dsafe), a viscosityversus temperature mapping, e.g., a table or graph, (not shown), a safepump displacement versus viscosity mapping (not shown), and/or otherdata. The viscosity versus temperature mapping may be determined basedon the type of oil used in the transmission, and the safe pumpdisplacement versus viscosity mapping may be determined experimentally.The control device 118 may send a signal to the pump 23 to increase thepump displacement by an amount determined using this stored data. Thecontrol device 118 may also send a signal to the engine 12 to increasethe engine speed by an amount determined using this stored data. Thememory 116 can also be used to store variables such as a threshold orfinal temperature (Tf) and/or a time limit (tf) that may be used todetermine when to end the oil warming strategy and to begin normaloperations of the work machine 8, a minimum increment for increasingpump displacement (dmin), a minimum increment for increasing enginespeed (vmin). The data stored in the memory 116 can be updated based onnew experimental determinations or based on changes in the type of oilused in the transmission.

INDUSTRIAL APPLICABILITY

The disclosed oil warming strategy may be applicable to any work machinethat includes a transmission such as a continuously variable orhydromechanical transmission. The disclosed oil warming strategy allowsoil in the transmission 10 to be warmed and accelerates the warmingprocess before stroking to maximum displacement the pump 23 that drivesthe motor 26 in the hydrostatic transmission 14. The operation of thetransmission 10 under this exemplary oil warming strategy will now beexplained.

FIG. 5 illustrates an exemplary flow chart of the exemplary oil warmingstrategy. Under cold start conditions the pump 23 (FIG. 2) is at zerodisplacement, the engine 12 produces zero output, and the oil in thetransmission 10 is generally at a low temperature (Step 200).Optionally, at this point, the temperature sensor 112 may be used tomeasure the temperature of the oil to determine if the temperature isbelow the threshold value (Tf) before proceeding.

As shown in FIG. 3, the first and second synchronizers 36, 38 engage thefirst and second low-speed reduction gears 64, 68, respectively, and theengine speed is increased so that engine 12 may idle at a low speed(Step 210) while the pump 23 rotates at zero displacement. The engine 12drives the input member 18, and the input member 18 delivers split inputpower to the hydrostatic transmission 14 and the planetary arrangement30. Specifically, the first and second fixed input gears 20 and 22simultaneously rotate upon rotation of the input member 18 and transferpower through the hydrostatic transmission input gear 24 and a firstplanetary input gear 102.

While the engine 12 is operating at low idle speed, the pump 23 uses thesplit input power to rotate at zero displacement and therefore does notdisplace any volume. As a result, power is not transferred to the motor26. Power is also not transferred to the hydrostatic output gear 28 anda second planetary input gear 104.

The split input power that is transmitted to the planetary arrangement30 provides hydromechanical output power, indicated by arrow 100, thatoutputs through the internal shaft 56 connected to the sun gears 50 ofthe first and second planetary gear sets 44 and 46, and through thesleeve 58, connected to the planet carrier 52 of the second planetarygear set 46. The second planetary output gear 62 intermeshes the secondhigh-speed reduction gear 70, which drives the first high-speedreduction gear 66. Accordingly, as the second planetary output gear 62rotates, the high-speed reduction gears 66, 70 also rotate. Likewise,the first planetary output gear 60 intermeshes the first low-speedreduction gear 64, which drives the second low-speed reduction gear 68.Accordingly, as the first planetary output gear 60 rotates, thelow-speed reduction gears 64 and 68 also rotate.

The first synchronizer 36 operates to engage the first low-speedreduction gear 64 to synchronize the speed, or revolutions per unittime, of the first rotating member 72 to the speed of the firstlow-speed reduction gear 64. In other words, to decrease relative speed,preferably to zero, between the first low-speed reduction gear 64 andthe first rotating member 72. When the speed of the first low-speedreduction gear 64 and the speed of the first rotating member 72 areequal, or substantially equal, the first low-speed reduction gear 64 andthe first rotating member 72 fully engage in a releasably lockedposition, as is known in the art.

At the same time, the second synchronizer 38 and the second low-speedreduction gear 68 are also in an engaged state, thereby synchronizingthe second rotating member 74 with the speed of the second low-speedreduction gear 68. In other words, the second synchronizer 38 operatesto decrease relative speed, preferably to zero, between the secondlow-speed reduction gear 68 and the second rotating member 74. When thespeed of the second low-speed reduction gear 68 and speed of the secondrotating member 74 are equal, or substantially equal, the secondlow-speed reduction gear 68 and the second rotating member 74 fullyengage in a releasably locked position, as is well known in the art.Arrows 106 of FIG. 3 indicate the flow of power through thehydromechanical transmission.

Although the first and second synchronizers 36, 38 are both engaged sothat the clutch discs 78 and 80 have a non-zero rotational speed, thereis a net zero, or approximately zero, torque on the final output member86 so that the work machine 8 may remain in a stationary, parkedcondition.

Both clutch assemblies 40 and 42 are disengaged so that the clutch hubs82 and 84 are disengaged from the rotating clutch discs 78 and 80,respectively. The clutch discs 78 and 80 rotate with a non-zero velocitywhile the clutch hubs 82 and 84 are grounded, thereby maintaining anon-zero clutch relative velocity. A drag (or frictional) force isgenerated in the oil between the clutch discs 78 and 80 and stationaryclutch hubs 82 and 84, and this drag force opposes the rotational motionof the clutch discs 78 and 80.

The drag force (and power) is higher when there is a higher clutchrelative velocity. Therefore, there is incentive to increase thedisplacement of the pump 23 because the clutch relative velocityincreases as the pump displacement increases, thereby generating higherdrag force.

The drag force is higher when the distance between the clutch discs 78and 80 and clutch hubs 82 and 84 is smaller. Since the gaps between theclutch discs 78 and 80 and respective clutch hubs 82 and 84 are small, ahigher drag force is generated. Also, the drag force is generated inboth clutch assemblies 40 and 42, thereby creating two locations forgenerating the drag force.

Heat is generated in both clutch assemblies 40 and 42 as a result of theapplication of the drag force on the oil in the clutch assemblies. Theheated oil can circulate throughout the transmission, e.g., by using oneor more pumps (not shown) that are driven by the engine 12. Therefore,unheated oil is continually replenished between the clutch discs 78 and80 and clutch hubs 82 and 84, and after being heated, the oil circulatesaway from the clutch assemblies 40 and 42, e.g., toward the hydrostatictransmission 14. Also, oil is circulated and warmed by the variousrotating gears in the transmission 10.

Although the clutch hubs 82 and 84 are disengaged from the correspondingclutch discs 78 and 80, a drag torque may be transmitted from therotating clutch discs 78 and 80 to the clutch hubs 82 and 84. The dragtorque may be produced in response to the drag force acting on the oiland urges the clutch hubs 82 and 84 in the direction of rotation of thecorresponding clutch discs 78 and 80.

When the clutch hubs 82 and 84 are urged into motion by thecorresponding clutch discs 78 and 80, rotational motion may betransmitted from the rotating members 72 and 74 to the output members 32and 34. The output shaft gears 94 and 96 and the ends of thecorresponding output members 32 and 34 intermesh with the final drivegear 98, and the final drive gear 98 outputs through the final outputmember 86 to the wheels or tracks. However, since the rotational forcesproduced by the drag torque acting on the clutch hubs 82 and 84 are inapproximately equal and opposite directions, as described below, theseforces produce a net torque of zero, or approximately zero, on the finaldrive gear 98 and the final output member 86.

The first and second low-speed reduction gears 64 and 68 are meshedtogether and spin in opposite directions. The rotational speeds of thegears 64 and 68 are also equal or approximately equal, since the sizesof the gears 64 and 68 are equal or approximately equal. When the firstand second synchronizers 36 and 38 engage with the first and secondlow-speed reduction gears 64 and 68, the rotational motion transferredto the clutch hubs 82 and 84 via the clutch discs 78 and 80 is equal, orapproximately equal, and opposite.

Thus, the rotational motion transferred to the output members 32 and 34and output shaft gears 94 and 96 is also equal, or approximately equal,and opposite. The output shaft gears 94 and 96 drivingly engage thefinal drive gear 98 in equal, or approximately equal, and oppositedirections, and therefore, the net torque transmitted to the final drivegear 98 is zero, or approximately zero. A net zero, or approximatelyzero, torque is transmitted to the wheels or tracks (not shown). As aresult, the oil is warmed while the engine 12 provides a non-zero outputand the work machine 8 remains in a stationary, parked condition.

The oil warming rate may be accelerated by increasing the engine speed(vengine) after a predetermined amount of time, e.g., 15 seconds (Step220). The engine speed may be increased by a reasonable predeterminedincrement (vmin), e.g., 200 revolutions per unit time. After thisinitial increase, the engine speed can be increased at regular timeintervals, e.g., every 60 seconds, by reasonable predetermined amounts,e.g., 200 revolutions per unit time.

Alternatively, instead of, or in addition to, increasing the enginespeed by a predetermined amount (vmin), the engine speed can beincreased to a highest safe engine speed (vsafe), which is determinedusing a closed-loop method based on experimentally-derived data storedin the memory 116. For example, the highest safe engine speed may bedetermined as a function of time, oil temperature, or oil viscosityusing a mapping stored in the memory 116. The engine speed value may beincreased to match the highest safe engine speed value (vsafe)determined using the stored data.

FIG. 6 a is a graph illustrating engine speed versus time for anexemplary disclosed oil warming strategy. The graph shows an example ofhow the engine speed can be stepped up over time.

By increasing the engine speed, more heat is generated in the clutchassemblies 40 and 42. When the engine speed increases, the relativevelocity across the clutch assemblies 40 and 42 also increases. Therotational speeds of the clutch discs 78 and 86 increases with respectto the clutch hubs 82 and 84. Thus, greater drag forces are applied tothe oil in the clutch assemblies 40 and 42, thereby creating more heatand accelerating the increase in oil temperature.

The process of warming the oil can also be accelerated by displacing thepump 23 from its zero displacement position (Steps 230-280). After theengine 12 has been at low idle for a predetermined amount of time, e.g.,15 seconds, the pump 23 can be displaced by a predetermined amount(dmin) to an intermediate position that is less than the maximumdisplacement (dmax), e.g., 20% displacement. After this initial increasein pump displacement, the pump displacement can be increased at regulartime intervals, e.g., every 60 seconds.

Alternatively, instead of increasing the pump displacement by apredetermined amount (dmin), the pump 23 can be displaced to the highestsafe pump displacement (dsafe) determined using a closed-loop methodbased on the data stored in the memory 116. The oil temperature ismeasured using the temperature sensor (Step 230). The highest safe pumpdisplacement is determined from the measured oil temperature using thedata stored in the memory 116. First, the oil viscosity is determinedusing the viscosity versus temperature mapping stored in the memory 116(Step 240). Then, the highest safe pump displacement (dsafe) isdetermined using the safe pump displacement versus viscosity mappingstored in the memory 116 (Step 250).

The highest safe pump displacement value (dsafe) determined using thestored data is compared to the minimum pump displacement value (dmin)that is also stored in the memory (Step 260). For example, if theminimum pump displacement value (dmin) is 20% displacement and thehighest safe pump displacement value (dsafe) is less than or equal to20% displacement (dsafe≦dmin), then the pump 23 is upstroked to 20%displacement (Step 270). However, if the safe pump displacement value isgreater than 20% displacement (dsafe>dmin), then the pump is upstrokedto the safe pump displacement value (Step 280).

The oil temperature (T) is measured using the temperature sensor 112(Step 290). If the temperature is greater than or equal to apredetermined final temperature (T≧Tf), then the oil warming strategy iscomplete (Steps 300 and 310). If the temperature is less than thepredetermined final temperature (T≦Tf), then it is determined whetherthe time limit has been exceeded (t≧tf), e.g., 3 minutes (Step 320). If3 minutes has elapsed since the warm-up started (t≧tf), then the oilwarming strategy is complete (Step 310).

However, if less than 3 minutes has elapsed since the engine was firstpowered (t<tf), then the transmission 10 continues running at itscurrent state for a predetermined amount of time, e.g, until 60 secondshas passed since the warm-up started (Step 330).

Then, the engine speed (vengine) may be compared to the highest safeengine speed (vsafe) determined in Step 220 (Step 340). If the enginespeed (vengine) is less than the highest safe engine speed (vsafe), thenboth the engine speed and the pump displacement are increased, as setforth in Steps 220-280. However, if the engine speed (vengine) isgreater than or equal to the highest safe engine speed (vsafe), then thepump displacement is increased, as set forth in Steps 230-280.

Thus, this is a “closed-loop” method because, as the oil warms, theengine speed and/or the pump displacement is increased incrementallyuntil the oil temperature reaches the final temperature.

FIG. 6 b is a graph illustrating pump displacement versus time for anexemplary disclosed oil warming strategy. The graph shows how the pumpdisplacement can be stepped over time, according to an embodiment of theinvention.

When the pump is upstroked to a non-zero displacement, oil may be warmedat the hydrostatic transmission 14. At a non-zero displacement, the pump23 uses the split input power from the input member 18 to fluidly drivethe motor 26 to convert the input power from the engine 12 tohydrostatic output power over a continuously variable speed ratio. Sincethe pump is rotating and displaced to the non-zero displacement, heat isgenerated in the hydrostatic transmission 14 and is transferred to theoil surrounding the hydrostatic transmission 14.

However, even though the motor produces hydrostatic output power, thework machine is still prevented from moving since the first and secondsynchronizers are both engaged and allow the clutch discs to rotate atapproximately equal and opposite speeds, thereby creating a net zerotorque on the final output member 86.

The hydrostatic transmission 14 outputs through the hydrostatic outputgear 28 to the planetary arrangement 30. Specifically, the hydrostatictransmission 14 outputs through the hydrostatic output gear 28 to asecond planetary input gear 104. The planetary arrangement 30 combinesthe hydrostatic output power from the second planetary input gear 104with the split input mechanical power to provide hydromechanical outputpower for application to a load, such as one or more driving wheels of avehicle, or tracks of an earth-working machine. The speed and torque ofthe planetary arrangement 30 can be infinitely varied by varying thestroke of the pump 23. When the pump 23 is at a non-zero displacement,greater speed and torque is applied to the planetary arrangement thanwhen the pump 23 is at zero displacement, thereby generating more heatto transfer to the oil.

It is to be understood that the exemplary oil warming strategy canprovide any combination of steps for increasing engine speed usingpredetermined increments or by using a closed-loop method and steps forincreasing pump displacement using predetermined increments or by usinga closed-loop method. The oil warming strategy may include only stepsfor increasing engine speed, or alternatively, only steps for increasingpump displacement.

Several advantages over the prior art may be associated with theexemplary oil warming strategy. The oil warming strategy allows heat tobe transferred to the oil without requiring stroking the pump 23 tomaximum displacement and without requiring full engagement of the clutchassemblies.

Since two synchronizers 36, 38 can be activated at the same time, thetorque produced by the engine 12 is effectively split evenly between thetwo clutch assemblies having clutch discs 78, 80 that rotate insubstantially equal and opposite directions. The oil warming strategyallows the oil in the transmission 10 to be warmed while the workmachine 8 remains in a stationary, parked condition.

The oil may be warmed by increasing the engine speed and/or byincreasing the pump displacement. To accelerate the warming process evenmore, both the engine speed and the pump displacement can be increased.An increase in engine speed results in an increase in relative clutchvelocity and increased drag force between the clutch hubs 82, 84 andclutch discs 78, 80. However, the increase in engine speed is limited bya maximum safe engine speed for preventing damage to the transmission10. An increase in pump displacement results in an increased motorspeed.

The oil warming strategy can be implemented automatically using aclosed-loop method, requiring minimal user interaction. Commands send tothe engine 12 or pump 23 for increasing the respective engine speed orthe pump displacement may be generated based on a predeterminedstrategy, e.g., by a fixed amount or an amount determined using amapping of engine speed or pump displacement as a function of a measuredoil temperature or viscosity. The mapping can be determined byexperimental data on the highest safe engine speeds and pumpdisplacements as a function of a measured temperature or viscosity ofthe oil in the transmission 10.

Furthermore, the oil warming strategy is capable of stoppingautomatically when the final temperature is reached. Therefore, thisstrategy can be used regardless of the temperature of the oil under thecold start conditions.

Heat may be generated in both clutch assemblies 78, 80, 82, 84 and inthe hydrostatic transmission 14, thereby increasing the rate at whichthe oil is warmed in the transmission 10 and decreasing the length oftime required for the oil to warm up to the final temperature. Themoving gears in the transmission 10 also serve to increase the heatgenerated. Furthermore, higher pump displacements not only increaseclutch drag power, but also increase variator power loss (losses in thepump 23 and the motor 26), thereby generating additional heat. Thevariator power loss loads the engine 12, which generates more heat.

Since the clutch discs 78, 80 rotate at approximately equal and oppositerotational speeds, a zero net torque is delivered to the output member86, thereby allowing the work machine 8 to remain in a stationary,parked condition.

Thus, the oil warming process may be accelerated while requiring minimaluser interaction. Since the user does not have to wait as long for theoil to warm up to begin normal operations of the work machine 8, userconvenience is increased.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the disclosed transmissionwithout departing from the scope or spirit of the disclosure. Otherembodiments of the disclosure will be apparent to those skilled in theart from consideration of the specification and practice of thedisclosure herein. It is intended that the specification and examples beconsidered as exemplary only.

1. A continuously variable transmission comprising: a fluid pump andmotor assembly coupled to an engine; and a mechanical transmissiondrivingly engaged to the fluid pump and motor assembly and the engine,the mechanical transmission comprising at least one gear, at least oneengaging device, and first and second clutch assemblies, each of theclutch assemblies comprising a clutch disc and a clutch hub, the atleast one engaging device being connected to at least one of the clutchdisc and the clutch hub, and the at least one engaging device isconfigured to engage the at least one gear so that rotational motion istransferable from the engine to both of the clutch discs; the clutchdiscs being rotatable at approximately equal and opposite speeds; andwherein the rotational motion of the clutch discs with respect to theclutch hubs produces a drag force on oil in the clutch assemblies. 2.The continuously variable transmission of claim 1, wherein the clutchassemblies transfer an approximately zero net torque to the outputmember.
 3. The continuously variable transmission of claim 1, whereinthe continuously variable transmission is a hydromechanicaltransmission.
 4. The continuously variable transmission of claim 1,wherein the at least one engaging device is positionable at a neutralposition for disengagement from the at least one gear and positionableat at least one engaged position for engagement with the at least onegear.
 5. The continuously variable transmission of claim 1, wherein theat least one gear includes at least one of a low-speed reduction gearand a high-speed reduction gear.
 6. The continuously variabletransmission of claim 5, wherein the at least one engaging deviceincludes a first engaging device configured to engage the low-speedreduction gear and a second engaging device configured to engage thehigh-speed reduction gear.
 7. The continuously variable transmission ofclaim 6, wherein the first engaging device and the second engagingdevice are configured to engage the respective low-speed and high-speedreduction gears at the same time.
 8. The continuously variabletransmission of claim 1, wherein the at least one engaging deviceincludes two synchronizing assemblies, one synchronizing assembly fixedto one of the clutch disc and the clutch hub of each clutch assembly. 9.The continuously variable transmission of claim 1, further including anauxiliary engaging device connected to an internal shaft of themechanical transmission and configured to engage an auxiliary gear ofthe mechanical transmission so that rotational motion is transferablefrom the engine to an output member of the mechanical transmission. 10.A continuously variable transmission configured to be driven by a drivesource, the continuously variable transmission comprising: a fluid pumpand motor assembly coupled to the drive source; a mechanicaltransmission drivingly engaging to the fluid pump and motor assembly andthe drive source, the mechanical transmission comprising first andsecond clutch assemblies, each of the clutch assemblies comprising aclutch disc and a clutch hub, rotational motion being transferable fromthe drive source to both of the clutch discs, the clutch discs beingrotatable at approximately equal and opposite speeds, wherein therotational motion of the clutch discs with respect to the clutch hubsproduces a drag force on oil in the clutch assemblies; and a controlsystem coupled to the drive source, the control system being configuredto increase a speed of the drive source, the mechanical transmissionbeing configured to output a substantially zero net torque as the speedof the drive source is increased.
 11. The continuously variabletransmission of claim 10, wherein the control system is configured toincrease the speed of the drive source in response to a predeterminedcondition.
 12. The continuously variable transmission of claim 11,further including a temperature sensor configured to measure atemperature of oil in the mechanical transmission, wherein thepredetermined condition includes at least one of a time limit and themeasured temperature being above a threshold temperature.
 13. Thecontinuously variable transmission of claim 10, wherein the controlsystem is configured to incrementally and periodically increase thespeed of the drive source, and the mechanical transmission is configuredto output the substantially zero net torque during each incrementalincrease of the speed of the drive source.
 14. The continuously variabletransmission of claim 10, wherein the control system is furtherconfigured to determine a safe speed of the power source based on storeddata for the power source and to increase the speed of the power sourceto the safe speed.
 15. The continuously variable transmission of claim14, wherein the control system is configured to compare the determinedsafe speed to an incremental increase in speed, and to determine adesired speed of the power source based on the comparison.
 16. Thecontinuously variable transmission of claim 10, wherein the controlsystem is coupled to the fluid pump and motor assembly, and the controlsystem is further configured to increase a displacement of a fluid pumpof the fluid pump and motor assembly, the mechanical transmission beingconfigured to output the substantially zero net torque while thedisplacement of the fluid pump is increased.
 17. A continuously variabletransmission configured to be driven by a drive source, the continuouslyvariable transmission comprising: a fluid pump and motor assemblycoupled to the drive source; a mechanical transmission drivinglyengaging the fluid pump and motor assembly and the drive source, themechanical transmission comprising first and second clutch assemblies,each of the clutch assemblies comprising a clutch disc and a clutch hub,rotational motion being transferable from the drive source to both ofthe clutch discs, the clutch discs being rotatable at approximatelyequal and opposite speeds, wherein the rotational motion of the clutchdiscs with respect to the clutch hubs produces a drag force on oil inthe clutch assemblies; and a control system coupled to a fluid pump ofthe fluid pump and motor assembly, the control system being configuredto increase a displacement of the fluid pump, the mechanicaltransmission being configured to output a substantially zero net torqueduring the increase of the displacement of the fluid pump.
 18. Thecontinuously variable transmission of claim 17, wherein the controlsystem is configured to incrementally and periodically increase thedisplacement of the fluid pump and the mechanical transmission isconfigured to output the substantially net zero torque during eachincrease of the displacement of the fluid pump.
 19. The continuouslyvariable transmission of claim 17, wherein the control system isconfigured to increase the displacement of the fluid pump in response toa predetermined condition.
 20. The continuously variable transmission ofclaim 19, further including a temperature sensor coupled to the controlsystem, the sensor being configured to measure a temperature of the oilin the mechanical transmission, the predetermined condition including atleast one of a time limit and the measured temperature being above athreshold temperature.
 21. The continuously variable transmission ofclaim 17, further including a temperature sensor coupled to the controlsystem, the sensor being configured to measure a temperature of the oilin the mechanical transmission, wherein the control system increases thedisplacement of the fluid pump to a substantially maximum displacementand, after the control system determines that the measured temperatureis above a final threshold temperature, the mechanical transmissionsubsequently outputs a substantially non-zero torque.
 22. Thecontinuously variable transmission of claim 17, wherein the controlsystem is further configured to determine a safe displacement based onstored data for the fluid pump and to increase the displacement of thefluid pump to the safe displacement.
 23. The continuously variabletransmission of claim 22, wherein the safe pump displacement is afunction of at least one of an approximate viscosity of the oil in themechanical transmission and a measured temperature of the oil in themechanical transmission.
 24. The continuously variable transmission ofclaim 22, wherein the control system is configured to compare thedetermined safe displacement to an incremental increase in displacement,and to determine the displacement of the fluid pump based on thecomparison.